The invention is directed to a method for fashioning ink channels in a write head for a dot-matrix ink printer means whereby form needles respectively cylindrically surrounded by piezo electric drive elements are aligned in a casting mold in accord with the desired course of the ink channels to be fashioned, are cast out with a casting compound, and, after the curing of the latter, are removed upon formation of the ink channels respectively partially surrounded by the allocated, piezo electric drive elements.
Known write heads for dot-matric ink printer means (see German Patent No. 25 43 551) that work based on the principle of individual drop ejection (drop on demand) contain individually drivable tubes of piezo ceramic material as drive elements for the ejection of ink drops from ink channels, each of these tubes of piezo ceramic material respectively surrounding an ink channel proceeding in the inside of the write head, surrounding the latter over a part of its length; given selection or drive of one of the drive elements, this tube of piezo ceramic material constricts, whereby an ink drop is ejected from the allocated ink channel.
For constructing such a write head, German Patent No. 25 43 451 discloses that the drive elements be plugged onto form needles and the needles be aligned in a casting mold in accord with the desired course of the ink channels; subsequently, the casting mold is filled out with a casting compound and the form needles are removed after the casting compound has cured. In order to prevent the drive elements from coming into direct contact with the writing fluid in the ink channels, which can lead to the penetration of the writing fluid into the porous ceramic of the drive elements and to shorts resulting therefrom, small protective tubes of metal are plugged into the drive elements before they are cast out and are drawn onto the form needles together with the drive elements, so that the small protective tubes in the finished write head isolate the drive elements that surround them from the interior of the allocated ink channels. The relatively thick wall thickness of the small protective tubes required in view of the inherent stability of the small protective tubes, however, requires a relatively high selection energy for the drive elements. Further, the relatively thick wall thickness of the small protective tubes complicates the fan-in of the ink channels to form a tight channel exit grid (nozzle grid), particularly when the drive elements should be arranged as close as possible to the channel discharges of the ink channels.
In a modified method likewise disclosed by German Published Application No. 25 43 420 for fashioning the ink channels in the write head, the drive elements are plugged onto the appertaining form needle upon interposition of a respective wire helix, so that the casting compound penetrates into the interspaces between the drive elements and the appertaining form needles in the subsequent casting and thus forms a protective layer isolating the appertaining drive element from the inside of the channel. However, the homogeneity of the protective layer is disturbed by the wire helix, so that it is not impossible that particles of the casting compound can crumble off when the form needles are removed or later when the drive elements are driven, namely due to the pressure pulses resulting therefrom.
In a further modified method disclosed by German Published Application No. 25 43 420 for fashioning the ink channels in the write head, whereby the drive elements are each coated with an insulating layer before their arrangement and alignment in the casting mold, a sudden transition in the inside channel wall between the insulating layer and the cast member can arise in the immediate proximity of the drive elements when the casting mold is cast out, whereby the course of the pressure pulse in the writing fluid is deteriorated when the drive elements are driven. Further, cracks can arise at this location.